In electrostatography an image comprising an electrostatic field pattern, usually of non-uniform strength, (also referred to as an electrostatic latent image) is formed on an insulative surface of an electrostatographic element by any of various methods. For example, the electrostatic latent image may be formed electrophotographically (i.e., by image wise photo-induced dissipation of the strength of portions of an electrostatic field of uniform strength previously formed on a surface of an electrophotographic element comprising a photoconductive layer and an electrically conductive substrate), or it may be formed by dielectric recording (i.e., by direct electrical formation of an electrostatic field pattern on a surface of a dielectric material). Typically, the electrostatic latent image is then developed into a toner image by contacting the latent image with an electrostatographic developer. If desired, the latent image can be transferred to another surface before development.
One well-known type of electrostatographic developer comprises a dry mixture of toner particles and carrier particles. Developers of this type are commonly employed in well-known electrostatographic development processes such as cascade development and magnetic brush development. The particles in such developers are formulated such that the toner particles and carrier particles occupy different positions in the triboelectric continuum, so that when they contact each other during mixing to form the developer, they become triboelectrically charged, with the toner particles acquiring a charge of one polarity and the carrier particles acquiring a charge of the opposite polarity. These opposite charges attract each other such that the toner particles cling to the surfaces of the carrier particles. When the developer is brought into contact with the latent electrostatic image, the electrostatic forces of the latent image (sometimes in combination with an additional applied field) attract the toner particles, and the toner particles are pulled away from the carrier particles and become electrostatically attached imagewise to the latent image-bearing surface. The resultant toner image can then be fixed in place on the surface by application of heat or other known methods (depending upon the nature of the surface and of the toner image) or can be transferred to another surface, to which it then can be similarly fixed or again retransferred to another surface upon which it is to be fixed.
Many well-known types of toner useful in dry developers comprise binder polymer materials such as vinyl addition polymers or condensation polymers. Such binder polymers are chosen for their good combinations of advantageous properties, such as toughness, transparency, good adhesion to substrates, and fusing characteristics, such as the ability to be fixed to paper at relatively low fusing temperatures while not permanently adhering to fusing rolls. As is well-known, vinyl addition polymers that are useful as binder polymers in toner particles can be linear, branched or lightly crosslinked. The most widely used condensation polymers are polyesters which are polymers in which backbone recurring units are connected by ester linkages. Like the vinyl addition polymers, polyesters useful as binder materials in toner particles can be linear, branched, or lightly crosslinked. They can be fashioned from any of many different monomers, typically by polycondensation of monomers containing two or more carboxylic acid groups (or derivatives thereof, such as anhydride or ester groups) with monomers containing two or more hydroxy groups.
While many binder polymers exhibit many desirable properties for use in electrostatographic toners, they also may have certain shortcomings. For example, binder polymers are commonly ground to a small particle size to provide the high degree of resolution required in good quality reproductions. Unfortunately, many polymers, and especially polyesters which are otherwise useful for toners, are not sufficiently easily ground to the very small particle sizes needed for high-resolution toners. To overcome this problem, methods have been developed which directly provide binder polymers having a controlled and predetermined size and size distribution suitable for use in electrostatographic toners by chemically prepared processes. One such method is a polymer suspension technique which is known in the prior art as a “limited coalescence” process, as described in U.S. Pat. Nos. 4,833,060, 4,965,131, 6,544,705, 6,682,866, and 6,800,412; incorporated herein by reference for all that they contain.
The preparation of toner polymer powders from a preformed polymer by the chemically prepared toner process such as the “evaporative limited coalescence” (ELC) offers many advantages over the conventional grinding method of producing toner particles. In this process, polymer particles having a narrow size distribution are obtained by forming a solution of a polymer in a solvent that is immiscible with water, dispersing, under suitable shear and mixing conditions, the solution so formed in an aqueous medium containing a fine particulate solid colloidal stabilizer to form stabilized dispersed droplets of the polymer solution, and removing the solvent. Removal of the solvent from the droplets provides solid binder polymer particles that are covered with a layer of smaller stabilizer particles. The resultant polymer particles are typically then isolated, washed and dried.
In the practice of this technique, polymer particles may be prepared from any type of polymer that is soluble in a solvent that is immiscible with water. The size and size distribution of the resulting particles can be predetermined and controlled by the relative quantities of the particular polymer employed, the solvent, the quantity and size of the water insoluble solid particulate suspension stabilizer, typically silica or latex, and the size to which the solvent-polymer droplets are reduced by mechanical flowing and shearing using rotor-stator type colloid mills, high pressure homogenizers, agitation etc. In a related alternative limited coalescence process, dispersed monomers may be polymerized in the presence of the particulate stabilizer to form solid binder polymer particles covered with the stabilizer particles.
US 2008/0176164 and US 2008/0176157 describe porous polymer particles that are made by a multiple emulsion process, that in one phase of the process results in formation of individual porous particles comprising a continuous polymer phase and internal pores containing an internal aqueous phase, where such individual particles are dispersed in an external aqueous phase. The LC process is used to control the particle size and distribution.
Porous toner particles in the electrophotographic process can potentially reduce the toner mass in the image area. Simplistically, a toner particle with 50% porosity should require only half as much mass to accomplish the same imaging results. Hence, toner particles having an elevated porosity will lower the cost per page and decrease the stack height of the print as well. The application of porous toners provides a practical approach to reduce the cost of the print and improve the print quality.
Toner particles made by the ELC or polymerization LC processes, whether porous or nonporous, are typically treated with base at pH 12 or greater to remove the colloidal stabilizers on the surface of the toner particles, when employing colloidal inorganic stabilizers in the preparation of the particles. This is necessary because the silanol end groups from colloidal silica stabilizer particles, e.g., interfere with the triboelectric properties of the carrier and toner particles employed as developers in electrostatogaphic imaging devices. As a result the surface fine colloidal silica particles are removed under high pH conditions. Such conditions, however, can prove to be costly and detrimental to certain binders that are easily hydrolyzed such as polyesters.
It is desirable to use high strength colorants for use in electrostatographic full process color printing processes that employ a toner set comprising a cyan, a magenta, a yellow and, optionally, a black toner. One such desirable colorant is the isoindoline yellow pigment known by its Color Index designation as Pigment Yellow 185, or PY 185. It confers good tinctorial strength and the required hue for many print applications. A problem with using PY 185 in a typical LC process, however, is that it is sensitive to high pH treatment required for removal of the colloidal stabilizer, and tends to wash out and change hue.
The reduction of attractive forces exerted on a toner enhances processes where the toner particles must move. Some processes that benefit from lower adhesive and cohesive forces are toner powder flow in the replenisher, mixing of toner in the developer station, development of toner onto the latent image, transfer of the image to intermediate and final receivers, and cleaning of residual images from photoconductors and intermediate receivers.
The attractive van der Waals forces between toner particles and other surfaces decrease as D/s2 where D is the toner diameter and s is the separation at the closest point between the toner and the other surface and s<<D. Relatively small separating particles may be added to the surface of toner particles to reduce attractive forces exerted on the toner particles. A few points of contact between the other surface and the toner created by the separating particles increase the separation between the surfaces. The points of contact of separating particles with the toner and another surface add a small attractive force. As such, the ideal situation is for the separating particles to be uniformly dispersed on the toner with a minimum coverage to affect the desired separation given the curvature of the toner and the size of the inorganic particles.
While the stabilizer particles employed in the LC processes are in the submicron range and if left alone on the surface can reduce the attractive forces exerted on the final dry toner, the inorganic stabilizer particles that are typically used in the limited coalescence processes such as silica particles unfortunately interfere with triboelectrification and must be removed from the binder polymer particles that are used in an electrostatographic toner as discussed above.
Consequently, surface forces of toners are typically modified by application of dry surface treatments to dry toner particles where inorganic stabilizer particles employed in the limited coalescence process have already been removed. The most common surface treatments are hydrophobically modified silicas, but fine particles of titanic, alumina, zinc oxide, tin oxide, cerium oxide, and polymer beads can also be used. The fine particles are chemically modified with silanes or polydimethylsiloxane to achieve the desired surface forces and triboelectric function. Varying particle sizes and amounts of surface treatment are used to ensure that the desired separation distance is maintained during violent collisions and shearing motion in toning stations to induce a static charge on the toner, to develop latent images on photoreceptors with toner, to transfer the developed images to intermediate and final receivers, and in other ancillary processes involving toner such as cleaning.
Violent collisions of the toner particle with carrier normal to the surface of the toner direct the impulse force on the surface treatment. The impulse force can exceed the strength of the toner core material (usually a melt adhesive polymer with a glass transition temperature, Tg, in the range of 50 to 60 degrees centigrade). The kinetic energy of the collision is transformed into heat and, because of the short duration of the collision event, the heat is localized at the surface treatment contact points with the toner particle and other surface. The local temperature at the contact briefly exceeds the Tg and the toner core material will plastically deform around the surface treatment increasing the area of contact. Because the separation in this area of contact is on the atomic scale, the attractive forces between the surface treatment and the toner are greatly increased. When this attractive force exceeds the shearing forces applied in the system, the surface treatment is tacked to the toner surface.
Before the fine particles used for surface treatment becomes tacked, shearing motions may move their position on the toner surface. The movement reduces the spacing and may allow contact of the core material of the toner particle with another surface. With sufficient shearing, the surface treatment will be concentrated in low (concave) areas of the toner surface necessitating an initial excess of surface treatment to obtain the desired separation. During gentle collisions and shearing contacts, some of the surface treatment may transfer to other surfaces. This reduces the effectiveness of the surface treatment and may create problems associated with the other surface. For example, transfer of the surface treatment to the carrier surface in a two component system may change the internal coefficient of friction resulting in changes in developer packed density and flow characteristics. Control of packed density of the developer is important because many toner concentration control algorithms rely upon changes in magnetic density as a function of toner concentration to measure the concentration for feedback control. Untacked surface treatment can also transfer to imaging surfaces ultimately accumulating and scumming these surfaces or other subsystems in contact with the imaging surface like roller chargers. Lastly, dry surface treatments have a portion of large agglomerates that if not properly dispersed can cause voids in the image.
There are many devices that have been developed to deagglomerate, disperse, and tack dry surface treatment agents on to the toner surface. Some mechanical devices such as the Cyclomix mixer generate an intense mechanical force by compressive shearing of a packed toner bed between a moving tools and a stationary wall. A high degree of shear rapidly heats the toner increasing the rate of tacking but also displacing some of the surface treatment into the low lying areas of the toner surface reducing the effectiveness of the surface treatment. Other devices such as the Henschel mixer rely upon toner-toner collisions in a fluidized bed to disperse the surface treatment. These collisions produce much lower shear and are more effective in achieving uniform dispersions. However, the normal forces are also lower and tacking is difficult to obtain. Also as the toner mass is decreased through reduced size or increased porosity the momentum of the particle is reduced leading to excessively long mixing times to achieve the desired firmly attached (tacked) state of the silica on the toner surface. The mixing time varies with toner mass and for toner particles less than 8 μm can frequently exceed 20 minutes. The effectiveness of such mixers and methods to quantify the degree of surface treatment tacking are described in U.S. Pat. No. 7,601,473.
U.S. Pat. No. 5,198,320 describes toner prepared by a limited coalescence process comprising binder polymer particles that have a surface layer of smaller polymeric stabilizer particles that are covalently bonded through a plurality of oxygen linkages to a polysiloxane oligomer containing pendant charge-agent moieties. The toner formation process involves a multistep process including the preparation of functionally active latex particles to stabilize the droplets in the ELC process and further the preparation of a polyorganosiloxane oligomer to functionalize the reactive groups of the latex. Surface treatment of such toner particles with hydrophobic inorganic spacing particles would need to be performed in a separate step, with the potential problems noted above.